1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device that can electrically read data or write data.
2. Description of the Related Art
Some of conventional semiconductor integrated circuits include one-time programmable memory (OTP memory) having relatively small capacity between several Kbits and tens of Kbits. Such an OTP memory is used for various applications such as storing information relating to a semiconductor memory device, a highly-integrated logic circuit, or a high-precision analog circuit formed together on the same chip (the information stored therein includes defect relief information, manufacturing information, adjustment information of circuit characteristics, individual chip intrinsic number, and the like).
As OTP memory employed in the above use, an OTP memory of gate-insulation film destruction type is known (for example, refer to JP2008-171477A). This OTP memory stores information based on whether a gate insulation film of a MOS transistor included in the memory cell is destroyed or not. That is, this memory cell stores “0” data in a state before the gate insulation film of the MOS transistor is destroyed (a high resistance state) while it stores “1” data in a state after the gate insulation film is destroyed (a low resistance state). Based on the electric property before and after the writing, the nonvolatile memory cell of the gate-insulation film destruction type is called an “antifuse element”.
This gate-insulation film destruction type antifuse element has the same structure as a normal MOS transistor used in other integrated circuits formed together on the same chip. Thus, a special process is not required to be added to the manufacturing process. Moreover, the production cost is low. Furthermore, since it is characterized in that it does not need any additional production step, it does not suffer from any deterioration of properties of major semiconductor elements mounted in combination on the same chip, such as fine-patterned memory elements for large capacity storage, high-speed transistors contained in high-speed logic circuits, and transistors for analog circuits exhibiting a high-precision electrical property. With these excellent characteristics, the antifuse element may be referred to as an optimal nonvolatile memory element for the above use.
However, there is another problem that must be solved for the use of the antifuse element of the gate-insulation-film destruction type. The problem is caused by a method for storing information used in the antifuse element of the gate-insulation-film destruction type. The method includes a step of destroying the device by applying a high voltage. In general, destruction of the gate insulation film creates a pinhole having a diameter of about 50 nm. However, it is difficult to control a position where the pinhole is created, and a shape thereof. For example, when a pinhole is created in the vicinity of a center of a channel of the transistor, a read current is small in comparison with the case that the pinhole is created in the vicinity of a source or a drain. This can be a cause of false reading of data stored in a memory cell.
As countermeasures of this problem, the channel length of the MOS transistor may be shortened, or amount of overlap between the source/drain diffusion regions and the gate electrode may be increased. But none of these countermeasures are enough for solving the problem. Moreover, a state of the gate insulation film after destruction tends to be unstable.
For example, when destruction of the gate insulation film is not enough, a current at the time of the reading becomes extremely small, or a large reading current is provided at an initial state, but it the current decreases rapidly as the time lapses.
In contrast, when the gate insulation film is destroyed excessively, a crystal structure of silicon just beneath the gate insulation film or a junction structure of the silicon may also be destroyed. This causes a decrease of the read current, and generation of an abnormal leak current. This is regarded as a problem. As a countermeasure for the problem, write a current flowing in the antifuse element after the destruction of the gate insulation film may be controlled. This countermeasure is not enough for solving the problem, either.
In this way, a conventional antifuse element of the gate-insulation-film destruction type does not fully solve a problem about false reading due to the information-storing method thereof.